Conventionally, in a process of fabricating a semiconductor with an ultra fine pattern, such as an LSI or a VLSI, a reduced projection exposure apparatus (hereinbelow, simply referred to as an “exposure apparatus”), which reduction-exposes a substrate coated with a photosensitive material to a circuit pattern drawn on a mask, thereby print-forming the pattern, is employed. In accordance with an increase in packaging density, further microminiaturization of a pattern is required, and handling such a pattern is required in the exposure apparatus.
To form an ultra fine pattern with the exposure apparatus, it is necessary to maintain high processing accuracy, such as focus accuracy, to bring an image formation surface (focus surface) of a reduced projection lens into correspondence with a subject (wafer) surface and an alignment accuracy for alignment of respective patterns through plural process steps. For this purpose, in the exposure apparatus, adjustable correction parameters are measured and set prior to substrate (wafer) exposure processing, and apparatus running is performed while the organized constituent elements (units) are controlled based on the parameters.
However, even when the correction parameters have been previously adjusted based on measurement, proper values (true values) of the correction parameters vary with an elapse of time, due to vibration during apparatus running, environmental change in atmospheric pressure, temperature, and the like, and thermal factors, such as exposure heat, and the like. That is, the correction parameters move away from the proper values. Accordingly, it is necessary to appropriately correct the correction parameter values by the amounts of fluctuation in the respective constituent elements.
Next, a conventional correction method for running control will be described. Prior to exposure of a substrate (wafer) by an exposure apparatus, an operator sets a recipe (operation parameters) for the exposure apparatus in accordance with a semiconductor device circuit pattern. The exposure apparatus sequentially performs exposure processing on the substrates (wafers), while controlling the constituents (units) in accordance with the recipe. FIG. 5 shows an example of a time chart of substrate (wafer) exposure by the exposure apparatus. In the figure, the lateral axis represents time, and a rectangular block, a brief segment of processing. In this example, while a substrate (wafer) is repeatedly exposed, the amount of fluctuation of an image formation surface position of a projection lens from a designed value (correction parameter) due to exposure heat of illumination on the reduced projection lens, or the like, is measured, and the image formation surface position is corrected by a predetermined number of wafers (three in FIG. 5).
When production by the exposure apparatus is started based on the recipe to measure the fluctuation of the image formation surface position of the projection lens (correction parameter) and corrects the position by three substrates (wafers), the fluctuation of the image formation surface position of the projection lens is measured prior to exposure of a first substrate (wafer), and the fluctuation-adjustable constituent elements (units) are properly controlled in accordance with the correction value. Next, substrate (wafer) exposure processing is repeated for three wafers, and again, the fluctuation of the image formation surface position of the projection lens is measured and correction is performed. Hereinafter, the measurement and correction are repeated by processing for three wafers. Thereby, the processing accuracy of the apparatus is maintained (for example, see Japanese Patent No. 3218631, paragraph 0063, which matured from Japanese Patent Laid-Open No. 5-021319).
In accordance with the development of recent ultra micro devices, the exposure apparatus must maintain higher accuracy. Today, several hundreds of correction parameters are known, and further, the number of correction parameters is increased for maintaining the apparatus accuracy. Further, allowable fluctuation amounts of the respective correction values are increasingly severe.
In this situation, in a case wherein the correction timing of the correction parameter is set with a predetermined number of wafers, as in the above-described conventional art, the difference between the correction parameter and a proper value might exceed a threshold value during wafer processing. In such a case, the quality of exposure in the wafer cannot be ensured, and the yield is degraded.
Further, the increase in the number of correction parameters increases the time required for measurement and correction of the parameters. As the fluctuation amounts become stricter, the measurement and correction phase must be repeated in a short period, and thus, the productivity of the exposure apparatus is degraded. Accordingly, it is important to prevent degradation of productivity of the exposure apparatus while maintaining accuracy of correction parameters. Further, to previously determine a correction period (correction timing) for each of the correction parameters by an operator, as in the above-described conventional art, requires a lot of time and effort.